The present invention generally relates to automatic repeat request in a wireless communication system, and more particularly to a Forward ACK channel for transmitting ACK/NACK indications responsive to data frames transmitted on a reverse packet data channel.
The purpose of a communication system is to transmit information from an information source to a destination over a communication channel. In wireless communication systems, noise and multipath fading cause bit errors to occur during transmission. Numerous error control techniques are used to combat interference and reduce bit errors. Most error control techniques introduce controlled redundancy into the information transmitted over the communication channel that can be used at the destination to detect and possibly correct bit errors that may occur during transmission. Two commonly used error control techniques are automatic repeat request (ARQ) and forward error correction (FEC).
The basic principle underlying ARQ is to add redundant bits or check bits to a message that allows detection of errors at the receiver. If the receiver detects errors in the received message, the receiver can request a repeat transmission of the message. Retransmissions are typically at the same data transmission rate as the original transmission. The number of retransmissions may be limited to a predetermined maximum number. ARQ is simple and achieves reasonable throughput when the error rate is low. Throughput diminishes, however, as the error rate increases because of the need to resend data.
FEC uses error-correcting codes to combat errors by adding redundancy to information before it is transmitted. The added redundancy enables the receiver to detect and correct most errors that occur during transmission. A receiver with knowledge of the error correcting code can detect and correct most bit errors. Examples of FEC codes include block codes, convolutional codes, and turbo codes.
Hybrid ARQ (HARQ) is another error control technique that combines ARQ and FEC. HARQ has been adopted for high-speed packet data (HSPD) channels to further enhance robustness against link adaptation errors. Using HARQ, messages are coded twice using an inner code and an outer code. The inner code may, for example, comprise a Cyclic Redundancy Check (CRC) code that is appended to the information bits prior to transmission to form a protected message. The protected message is then coded using FEC, for example, a convolutional code or turbo code associated with FEC. Both the information bits and CRC bits are coded. The coded message is then transmitted to a receiving terminal, which decodes the message and performs a CRC check. If the number of errors in the message is within the capabilities of the error correction code, the errors will be corrected without the need for retransmission. Only if the number of errors in the message exceeds the capabilities of the error correcting code will retransmission be requested.
When ARQ or HARQ are used on a reverse link channel, there needs to be some method of transmitting ACK/NACK indications from the base station to the mobile terminal. Conventionally, ACK/NACK indications are transmitted using binary phase shift keying (BPSK). With BPSK, the phase of the signal is varied to ACK or NACK a mobile station transmission on the reverse link channel.